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Questions What is the cause of these observed seasonal variations? –Snow and ice loading? –Systematic errors in orbits, tidal loading, etc.? Do other sites (used to realize the ITRF) include seasonal variations not included in the ITRF model? –In standard time series, these sites have smaller seasonals than sites not used to realize the frame. Does the error go into the frame? –Does the method of frame realization affect seasonal signals at local/regional sites of interest?

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Regional realization vs. ITRF ITRF solutions uses –Minimum constraints –“Free” velocity solution –Velocity and frame constraints added –Seasonal variation affects frame only if it biases mean velocities Regional solution or time series alignment –Daily or weekly realization of ITRF using frame sites –Positions may not be linear in time, cannot assume linear velocity of all sites in solution –Seasonal variation in frame sites may impact frame realization, and thus regional time series

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Time Series – IRKT (Irkutsk) Original – Linear ITRFAfter final iteration

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Impact On User Sites Generate 1 year synthetic time series –Seasonal variations from final iteration at all reference frame realization sites –NO seasonal variations at test sites –Realize ITRF each day assuming no seasonal variation at frame sites Test sites show seasonal variations –1-2 mm amplitude horizontal –2-4 mm amplitude horizontal –These variations are purely the result of a seasonal bias in the frame realization

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Error Induced in Time Series Fairbanks, AlaskaWestern Aleutians

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Conclusions Seasonal variations at many sites exceed 5 mm/yr amplitude – 2 ppb peak to peak –At most northern sites, pattern is consistent with snow and ice loading Seasonal variations bias daily or weekly frame realization –Frame parameters and station coordinates –Biases become embedded in time series Be careful in deciding what is the “data” A challenge for assessing time-dependent deformation Seasonals should be assessed at a global scale, perhaps with ITRF2005 residuals